From 1c37997fa70129efea84c02cf4d863a44660ab2e Mon Sep 17 00:00:00 2001
From: Urgau <urgau@numericable.fr>
Date: Sun, 19 Nov 2023 21:41:55 +0100
Subject: [PATCH] Improve diagnostic for unexpected feature config name from
 Cargo

---
 compiler/rustc_lint/src/context.rs      |  2 ++
 tests/ui/check-cfg/cargo-feature.rs     | 14 ++++++++++++++
 tests/ui/check-cfg/cargo-feature.stderr | 11 +++++++++++
 3 files changed, 27 insertions(+)
 create mode 100644 tests/ui/check-cfg/cargo-feature.rs
 create mode 100644 tests/ui/check-cfg/cargo-feature.stderr

diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index d500c3b66d6..ad2047c1903 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -739,6 +739,8 @@ pub trait LintContext {
                         } else {
                             db.span_suggestion(name_span, "there is a config with a similar name", best_match, Applicability::MaybeIncorrect);
                         }
+                    } else if name == sym::feature && std::env::var_os("CARGO").is_some() {
+                        db.help("consider defining some features in `Cargo.toml`");
                     } else if !possibilities.is_empty() {
                         let mut possibilities = possibilities.iter()
                             .map(Symbol::as_str)
diff --git a/tests/ui/check-cfg/cargo-feature.rs b/tests/ui/check-cfg/cargo-feature.rs
new file mode 100644
index 00000000000..ea48c6ea201
--- /dev/null
+++ b/tests/ui/check-cfg/cargo-feature.rs
@@ -0,0 +1,14 @@
+// This test checks that when no features are passed by Cargo we
+// suggest adding some in the Cargo.toml instead of vomitting a
+// list of all the expected names
+//
+// check-pass
+// rustc-env:CARGO=/usr/bin/cargo
+// compile-flags: --check-cfg=cfg() -Z unstable-options
+// error-pattern:Cargo.toml
+
+#[cfg(feature = "serde")]
+//~^ WARNING unexpected `cfg` condition name
+fn ser() {}
+
+fn main() {}
diff --git a/tests/ui/check-cfg/cargo-feature.stderr b/tests/ui/check-cfg/cargo-feature.stderr
new file mode 100644
index 00000000000..619410a28f3
--- /dev/null
+++ b/tests/ui/check-cfg/cargo-feature.stderr
@@ -0,0 +1,11 @@
+warning: unexpected `cfg` condition name: `feature`
+  --> $DIR/cargo-feature.rs:10:7
+   |
+LL | #[cfg(feature = "serde")]
+   |       ^^^^^^^^^^^^^^^^^
+   |
+   = help: consider defining some features in `Cargo.toml`
+   = note: `#[warn(unexpected_cfgs)]` on by default
+
+warning: 1 warning emitted
+